Field of Invention
The present invention relates to cavitation physics and ultrasound imaging technologies, wherein a method of the present invention combines array transducer plane-by-plane wide beam cavitation detection, wide beam minimum variance adaptive beamforming, and Nakagami parametric imaging algorithms, so as to achieve microsecond-resolution cavitation three-dimensional spatial-temporal distribution imaging and cavitation bubble concentration quantitative imaging under conditions of a steady-state free field and a pulsating flow.
Description of Related Arts
Cavitation refers to a series of kinetic processes such as micro nucleate oscillation, growth, shrinkage and even collapse, which is caused by activating cavitation nucleus in liquid by external energy (heat/force). In biomedical field, the cavitation is a main mechanism for drug delivery, gene transfection, extracorporeal lithotripsy, thrombolysis, hemostasis, hyperthermia, thermal ablation of tumors, etc. Cavitation processes comprises the following phases: cavitation nucleation, cavitation bubble linear and nonlinear vibration, cavitation bubble growth, rapidly shrinking to collapse, and cavitation bubble dissipating. The cavitation is divided into steady-state ones featuring non-inertial cavitation, and transient-state ones featuring inertial cavitation. In a liquid medium, a minimum cavitation energy value is called a cavitation starting threshold, which depends on the static pressure, initial temperature and structural state of the liquid medium and diverse cavitation nuclei externally added into the liquid. Therefore, the cavitation generated in the liquid medium has certain randomness, but with the same environments and energy, shapes and distributions of cavitation bubble clouds are repeatable. Conventionally, in order to better study cavitation generation mechanisms based on different media, so as to better control and use the cavitation, effective cavitation detection and imaging methods should be researched.
Conventionally cavitation detection and imaging are mainly achieved by optical and acoustic methods. Optical detection imaging comprises high-speed/ultra-high-speed photography, sonoluminescence and sonochemiluminescence, which is able to photograph and observe cavitation bubble behavior and spatial-temporal dynamic characteristics. Advantages thereof are being intuitive, good synchronization, and high time resolution. Shortcomings are that on one hand, media translucent is highly demanded and the optical detection imaging is not suitable for in-situ study; on the other hand, images obtained are information superimposed along an optical penetration direction. Acoustic detection method is based on sound information about cavitation processes or cavitation micro bubbles, such as harmonics, sub harmonics, super harmonic and broadband noise, wherein passive cavitation detection (PCD) and active cavitation detection (ACD) are most widely used. The PCD passively receives acoustic scattering signals generated by the cavitation through a transducer, and the ACD detects potential cavitation areas with low-voltage pulse echoes. However, both the PCD and the ACD generally use a single element transducer, which are not able to provide spatial distribution of cavitation micro bubble due to limited space detection areas.
Based on the PCD and the ACD, a two-dimensional array transducer is used as a cavitation detection transducer, resulting in development of passive cavitation imaging (PCI) and active cavitation imaging (ACI). Due to transient characteristics, vibration, collapse and dissipation of ultrasonic cavitation micro bubbles only last for a few microseconds. Therefore, the cavitation imaging method requires microsecond time resolution. Meanwhile, for the cavitation transient characteristics, it is necessary to obtain spatial-temporal distribution of cavitation micro bubble, comprising sequence spatial-temporal cavitation distributions of different cavitation energy source actuation durations and cavitation dissipating times. The PCI passively receives through an array transducer and then obtains cavitation bubble two-dimensional distribution through channel signal source reconstruction, wherein a reconstruction algorithm is complex and spatial resolution is not high. The ACI comprises conventional B-mode imaging and ultra-fast active cavitation imaging methods. Since the B-mode imaging is achieved through line by line scanning, time differences exist between different scanning lines of the same frame, and a time resolution is not at a microsecond level. The ultra-fast active cavitation imaging launches a plane wave, so sensitivity and lateral resolution thereof need to be improved, and a time resolution is a few hundred microseconds, which is not able to meet requirements of studying cavitation transient distribution.
Based on the cavitation imaging, cavitation bubble characteristics need to be identified, comprising cavitation quantifying, and cavitation size and concentration distributions. Conventionally, cavitation quantification methods mainly adopt inertial cavitation doses and non-inertial cavitation doses, which calculate a root mean square value of broadband noise within a specific frequency band or a sub harmonic amplitude for obtaining a relative measure of cavitation intensity, so as to respectively measure the relative intensity of transient-state and steady-state cavitation. However, for one-dimensional radio frequency data collected by the PCD, such quantification methods cannot describe cavitation intensity distribution. Conventional cavitation concentration detection methods comprise a laser phase Doppler method which is focused on size distributions of cavitation bubble, but concentration distributions of cavitation bubble at different spatial locations have not been searched, resulting in no spatial information provided.
Conventionally, cavitation detection and imaging methods are limited to one-dimension and two-dimension. Actually, distribution areas of cavitation bubbles are throughout a whole focal region or even greater. Furthermore, in clinical applications such as focused ultrasound therapy, there may be other organization media in an acoustic propagation path, in such a manner that sound field distribution changes will lead to asymmetry. Therefore, it is necessary to develop microsecond-resolution cavitation three-dimensional spatial-temporal distribution imaging and cavitation bubble concentration quantitative imaging methods. In addition, there are fewer researches on cavitation under flow conditions, especially pulsating flow conditions. However, blood flow of a human body is pulsating flow, so it is necessary to study three-dimensional spatial-temporal cavitation distribution based on such conditions, especially three-dimensional cavitation distribution at different time points within a pulsating flow cycle.